The present invention relates to phase-locked loops and, more particularly, to a novel phase-locked loop having greatly reduced frequency modulation of the output frequency waveform over the periond of the synchronizing lower reference frequency waveform.
Phase-locked loops are typically utilized to generate a higher-frequency output waveform synchronized to a lower-frequency reference waveform. The higher-frequency waveform, typically generated by a voltage-controlled oscillator, is frequency-divided by some factor N to provide a counted-down frequency equal to the reference frequency. The two frequencies are phase detected to derive a frequency-error signal. The frequency-error signal from the phase detector output must be filtered, typically with a capacitor external to the phase detector, in order to reduce the phase detector output ripple and minimize frequency modulation of the locked oscillator. If a relatively high order of frequency division N is utilized, a small amount of phase detector output ripple will result in a relatively large degree of locked-oscillator frequency modulation. Often, the amount of oscillator frequency modulation is unacceptable. Hitherto, the phase detector output filter capacitance has been increased to reduce the degree of this frequency modulation, although there is a practical limit to the magnitude of the filter capacitance. A filter capacitor having too large a capacitance value tends to cause the loop to become underdamped and have a tendency to oscillate, while the lock-in range of the loop becomes very small and renders synchronization to a reference signal difficult, if not impossible. It is therefore highly desirable to operate on the phase detector output waveform (typically a square wave at twice the frequency of the reference input, at lock or near-lock conditions) to reduce the degree of controlled oscillator frequency modulation, while avoiding the synchronization and stability problems typically presented by a large filter capacitance at the phase detector output.